Neuroscience & Medicine, 2013, 4, 223-252
Published Online December 2013 (http://www.scirp.org/journal/nm)
http://dx.doi.org/10.4236/nm.2013.44035
Open Access NM
223
A New Generation of Drugs: Synthetic Peptides Based on
Natural Regulatory Peptides
Timur Kolomin1*, Maria Shadrina1, Petr Slominsky1, Svetlana Limborska1, Nikolay Myasoedov2
1Department of Molecular Basis of Human Genetics, Institute of Molecular Genetics Russian Academy of Sciences, Moscow, Russia;
2Department of Chemistry of Physiologically Active Compounds, Institute of Molecular Genetics Russian Academy of Sciences,
Moscow, Russia.
Email: *kotimur@img.ras.ru
Received July 15th, 2013; revised August 13th, 2013; accepted September 10th, 2013
Copyright © 2013 Timur Kolomin et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
Natural regulatory peptides are biologically active compounds that are produced by various cells and provide a link
among the main regulatory systems of the body. The field of research into the biologic activity of endogenous regula-
tory peptides is extremely vast. These peptides affect the cardiovascular, immune, reproductive, endocrine, digestive,
and other systems, alter energy metabolism, and are especially effective in the regulation of the central nervous system.
Despite of the wide range of preventive and therapeutic effects of natural regulatory peptides and proteins, their applica-
tion in clinical practice is difficult. This is primarily because of their extreme instability, as they are rapidly degraded by
proteases of the gastrointestinal tract, blood, cerebrospinal fluid, and other biologic media. Compounds with higher sta-
bility (i.e., a considerably longer half-life compared with that of natural molecules) and the ability to provide a direc-
tional effect on the various body systems were obtained from modifications of endogenous regulatory peptides. Syn-
thetic analogs of regulatory peptides, as a rule, contain only natural amino acids in their composition, and their biodeg-
radation does not lead to the formation of toxic products; thus, they have fewer side effects. This review focuses on the
consideration of two synthetic regulatory peptides, Semax and Selank, which were the bases for the creation of new
drugs that are used effectively in the treatment of various diseases of the nervous system. The synthetic analog of an
adrenocorticotropic hormone 4 - 10 fragment (ACTH4-10) Semax is a powerful neuroprotective agent that is particularly
effective as a therapy for stroke. Selank was synthesized on the basis of the natural immunomodulator tuftsin. Selank is
a powerful anxiolytic that is used as a therapy for generalized anxiety disorder and neurasthenia without sedative and
muscle-relaxant effects. This review presents the results of research aimed at studying the influence of these peptides on
the transcriptome of brain cells. The problems of drugs developed based on the clinical activities of Semax and Selank
are discussed separately.
Keywords: Semax; Selank; Regulatory Peptides; ACTH; Tuftsin; Neuroprotection; Nootropes; Anxiolytics
1. Introduction
The development and investigation of novel neurotropic
drugs seem to be a challenging scientific problem for
modern biology and pharmacology. Effective and safe
pharmacological remedies that stimulate the cognitive
functions of the brain and are neuroprotective in different
pathologies of the central nervous system are needed
urgently. The use of endogenous regulators of the body,
especially regulatory peptides, as a basis for the devel-
opment of drugs that meet the requirements stated above
provides one of the most promising directions for this
type of research. Peptide substances have been shown to
be the direct regulators or mediators of the majority of
physiological processes that occur in the body. In addi-
tion, regulatory peptides exhibit a vast range of biologic
activities.
Neuropeptides represent important regulators of the
central nervous system. However, the study of the bio-
logic properties and mechanisms of action of the major
known neuropeptides revealed that the clinical applica-
tion of the full-size molecules of these compounds was
extremely complicated because of their instability inside
the body and the fugacity of their effects. In this context,
it becomes necessary to use them as a basis of the de-
*Corresponding author.
A New Generation of Drugs: Synthetic Peptides Based on Natural Regulatory Peptides
224
velopment of more stable synthetic molecules by various
modifications, which might be more effective compared
with their prototypes, and at the same time, less harmful
for use in clinical settings.
The aim of this review is to consider the characteristics
of two new-generation peptide preparations: the synthetic
analog of an adrenocorticotropic hormone 4 - 10 frag-
ment (ACTH4-10), the neuroprotector and nootrope Se-
max, and the natural immunopeptide tuftsin analog, the
anxiolytic and nootrope Selank. Large-scale studies of
the range of biologic activity and mechanism of action of
Semax and Selank revealed that the therapeutic potential
of drugs derived from these compounds was far from
being exhausted, and that new indications for their ap-
plication could be discovered.
Success in the creation of drugs based on the synthetic
peptides Semax and Selank and the achievement of posi-
tive outcomes from their clinical applications required
the study of their mechanisms of action, as this opens
new approaches to the creation of pharmacologically
important molecules and reveals the impact of new tar-
gets for the directed regulation of cellular functions in
health and disease.
2. Natural and Synthetic Regulatory
Peptides
Regulatory peptides are universal endogenous biologic
regulators of cellular functions in the body. They are part
of the most complicated system of specialized signaling
molecules, of which the principal function is the integra-
tion of the nervous, endocrine, and immune systems to
form a united functional continuum.
Extensive investigation of regulatory peptides began in
the 1970s after a team of Dutch researchers headed by
David de Wied demonstrated that the adrenocorticotropic
hormone and its fragments affected the learning of ani-
mals [1]. Currently, over 9000 regulatory peptides that
differ in both structure and properties have been de-
scribed, the majority of which contain from 2 to 50
amino acid residues and belong to the group of oligopep-
tides with no more than 20 amino acids or to the group of
polypeptides with 20 to 100 amino acid residues.
Posttranslational processing is one of the main path-
ways of regulatory-peptide formation in the body, via
which they are isolated from the physiologically inactive
precursor proteins with the help of specialized enzymes-
proteases. In some cases, a single precursor protein gives
rise to a whole group of peptides, which are required for
the successful accommodation of the body to various
well-definite changes in the environment. This type of
pathway is the most effective way to achieve this goal
and provides a practically immediate release of the
amount of regulatory peptides required by the body.
Moreover, the processed peptides have a far shorter half-
life compared with that of their precursor proteins, which
is a significant precondition for the rapid interruption of
the signal processes mediated by these molecules, in case
they become unnecessary [2].
The mechanism of action of regulatory peptides in a
target cell is conditioned by their binding to special
membrane receptors. All regulatory peptides are plei-
otropic and are able to influence a number of physio-
logical functions, with individual functions being simul-
taneously regulated by several peptides. The fact that
regulatory peptides that affect target cells are able to
regulate the release of other regulatory peptides, which,
in turn, initiate the generation of a new wave of regula-
tory peptides, represents an important feature of their
mechanism of action. This allowed Ashmarin et al. to
pinpoint the existence of cascade processes in the regu-
latory peptide system that sustain the effect of a single
administration of a peptide for a considerable period (up
to several days), whereas the half-life of the peptide does
not exceed several minutes. This sets forth the functional
continuum of regulatory peptides that participate in the
transfer of information among the systems of the body
(its organs, tissues, cell groups, and individual cells),
thereby regulating their activity and integrating their
performance into a unity. Thus, the regulatory-peptide
system is involved in the regulation of virtually all
physiological responses of the body, and maintains the
homeostasis of all of its systems [3-6].
All cells of the body permanently synthesize, and
support the required levels of, regulatory peptides. How-
ever, their biosynthesis undergoes changes in the pres-
ence of broken homeostasis. A similar instability occurs
during adaptive responses to the background of physical
or/and emotional strain, predisease conditions, etc.,
thereby preventing the disturbance of the functional bal-
ance, which is an indication that the regulation of the
metabolism of endogenous peptides occurs at the cellular
level (transcription, translation, and posttranslation proc-
essing), at the tissue level (secretion and inactivation of
regulatory peptides), and at the level of the body as a
whole [7].
The classification of regulatory peptides takes into ac-
count their chemical and physiological structure and their
origin. One of the main difficulties in classifying these
molecules lies in their multifunctional character, which
does not allow one or even several functions to be estab-
lished in a substrate. Considering the physiological and
biochemical properties of regulatory peptides, previous
studies have suggested a three-group classification [8]:
Class I regulatory peptides exert a distant effect and
have high affinity for receptors; in addition, their
principal characteristics render them similar to oli-
gopeptide hormones, such as vasopressin, oxytocin,
somatostatin, and endogenous opiates.
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A New Generation of Drugs: Synthetic Peptides Based on Natural Regulatory Peptides 225
Class II regulatory peptides exert a distant effect and
have relatively low affinity for receptors. They are
synthesized from nonspecific proteins (collagen and
elastin). Glyprolines and enterostatins, for example,
belong to this class.
Class III regulatory peptides comprise peptides that
act locally, at the level of organs and tissues (e.g.,
neurotrophins).
Numerous studies on the biologic properties and
mechanisms of action of endogenous regulatory peptides
suggested the potential application of peptide regulators
in clinical settings. However, regardless of their preven-
tive and therapeutic action, the use of peptide regulators
as medicinal drugs appeared extremely difficult, primar-
ily because of their multifunctional nature and high in-
stability. After introduction into the body, regulatory
peptides undergo rapid splitting by the proteases of the
alimentary tract, blood, cerebrospinal fluid, and other
biologic media. Therefore, the desired effects of regula-
tory peptides introduced into the body are transient and
limited [9].
Thus, the search for methods to enhance the efficiency
and duration of the effect of regulatory peptides became
the main task of researchers in this field. Among the nu-
merous attempts undertaken to solve this problem, the
replacement of natural left-handed amino acids in the
regulatory peptide molecule by D-isomers or even or-
ganic radicals was one of the main approaches used in
the creation of synthetic analogs of endogenous regula-
tors. In the majority of cases, this method caused a con-
siderable increase in the resistance of the compounds
obtained to protease action. Nevertheless, the addition of
non-natural components to natural molecules resulted in
negative side effects for the synthetic analogs [10].
The inclusion in the structure of the natural molecules
of amino acid sequences that are resistant to peptidases
was another approach that was used to solve this problem.
The introduction of proline-residue-enriched regions into
natural molecules represented an alternative approach. It
is common knowledge that the organs and tissues of the
majority of hematothermal animals bear a variety of
low-specificity exo- and endopeptidases that do not break
AA-Pro bonds, in which AA is any amino acid followed
by sequences rich in proline residues. Small amounts of
specific proline-hydroxylases accumulate mainly in indi-
vidual organs and tissues. Based on this fact, it was hy-
pothesized that the attachment of proline-carrying se-
quences to the C-terminal region of natural molecules
would elicit a prolonged action and a stronger effect
compared with those of the natural entity [11].
Members of the glyproline family were the most effec-
tive among the proline-carrying sequences used in the
modification of natural regulatory peptides. Glyprolines
are oligopeptides that are formed during the degradation
of collagen and elastin, and contain glycine and proline
(Gly-Pro, Pro-Gly, Pro-Gly-Pro, etc.). Glyprolines are
extremely stable, and protect the sequences added to
them against cleavage by the proteases of the body [12,
13].
A number of synthetic medical preparations based on
natural regulatory peptides have been developed by a
research team headed by Drs N.F. Myasoedov and I.P.
Ashmarin using the approach described above. Among
them, the two most promising peptide drugs, Semax and
Selank, deserve special attention.
Semax (H-Met-Glu-His-Phe-Pro-Gly-Pro-OH) is a
synthetic analog of the adrenocorticotropic hormone 4 -
10 fragment (ACTH4-10). It exhibits distinct neuroprotec-
tive and nootropic properties and is the basis for a num-
ber of drugs that are used in clinical practice for the
treatment of CNS diseases (ischemic brain stroke, dys-
circulatory encephalopathy, optic nerve atrophy, etc.) and
to enhance adaptability under extreme conditions in heal-
thy persons.
Selank (H-Thr-Lys-Pro-Arg-Pro-Gly-Pro-OH) is a
synthetic analog of the short fragment of the human im-
munoglobulin G heavy chain (tuftsin). It exhibits anti-
anxiety and nootropic effects. A drug based on Selank is
used in therapy for generalized anxiety disorders and
neurasthenia and to enhance adaptability under extreme
conditions in healthy persons.
Despite the fact that all drugs developed from these
peptides have passed all preclinical and clinical tests and
are successfully used in clinical practice, the range of
their physiological activity and their mechanisms of ac-
tion in the body have not been studied fully and large-
scale research on these peptides is still in progress.
3. Semax Is a New-Generation
Neuroprotective and Nootropic
Compound
3.1. Background
The adrenocorticotropic hormone (ACTH) and its frag-
ments represent one of the most actively studied classes
of endogenous regulatory peptides. ACTH is a single-
chain polypeptide consisting of 39 amino acids that is
formed in the glandular lobe of the anterior lobe of the
hypophysis via posttranslational processing of the pre-
cursor protein proopiomelanocortin (POMC) [14,15].
The main function of ACTH is associated with steroi-
dogenesis. After binding to the melanocortin 2 (MC2)
receptor on the plasma membranes of cortical cells of the
suprarenal gland, corticotropin enhances the synthesis
and secretion of glucocorticoids and intensifies the trans-
formation of cholesterol to pregnenolone. The 24 N-ter-
minal amino acids of ACTH are required to attain the full
manifestation of the biologic activity of this hormone
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A New Generation of Drugs: Synthetic Peptides Based on Natural Regulatory Peptides
226
[16].
ACTH is one of the main regulatory components of
the pituitary-hypothalamic axis, and interacts with other
peptide hormones (prolactin, vasopressin, thyroliberin,
the vasointestinal peptide, opioid peptides, etc.) and with
the mediatory systems of the hypothalamus [7]. In addi-
tion to its hormonal effect, ACTH displays a broad range
of extrahormonal activities. For instance, multiple studies
have shown that ACTH and its fragments affect human
and animal behavior. The first evidence of the effect of
ACTH on the behavior of animals was obtained in the
mid-1950s [17,18]. At that time, David de Wied and col-
leagues studied the influence of ACTH and its fragments
on the learning abilities of animals and showed that the
removal of the glandular lobe of the hypophysis results in
disturbed formation of conditioned responses. ACTH
administration to hypophysis-ectomized animals com-
pletely normalized the learning process [19]. It was then
found that ACTH accelerates learning processes in intact
animals [20], and that its action is independent of ACTH
hormonal activity and likely results from a direct action
on the CNS [21].
Numerous studies showed that the N-terminal region
of the ACTH molecule is the principal region responsible
for the behavioral activity of the hormone and that the
ACTH4–10 peptide is the minimal fragment retaining the
behavioral effect of the full-length ACTH molecule, with
complete loss of its hormonal activity [22-24]. It was
shown in these cases that the Phe residue located at posi-
tion 7 played the key role in the manifestation of the be-
havioral effects of ACTH fragments [25].
The study of the nootropic effects of different ACTH
fragments showed that ACTH1-10 and ACTH4-10 influ-
enced the learning of hypophysis-ectomized animals as
successfully as ACTH1-24 did [26], and the positive ac-
tion of ACTH4-10 on conditioned-response formation in
intact animals was subsequently shown. The ACTH4-7
and ACTH5-10 fragments also accelerated the formation
of conditioned responses, with ACTH4-7 being the most
active of all the peptides studied [27,28].
Clinical trials performed on volunteers proved that
ACTH exerted a positive action on human cognitive
abilities and improved attention and the ability to per-
form prolonged and monotonous work [29,30].
No uniform opinion regarding the mechanisms of
ACTH action has been formed to date, and it is presumed
that the influence of ACTH-family peptides on learning
processes is caused by increased circulation of monoam-
ine in the brain. It has been shown repeatedly that in-
creased circulation and content of catecholamines and
serotonin in different brain regions occurs after admini-
stration of ACTH and its fragments. A nootropic effect is
also likely to occur because of the increased metabolism
of acetylcholine in the hippocampus and cerebral cortex
[31].
In parallel with the nootropic effect of ACTH and its
fragments, neuroprotective and neurotrophic effects were
also shown. Studies performed on cultured neurons
proved that ACTH exerted a trophic action in these cells
and that even small doses (10 nmol) of the hormone
stimulated the growth of nerve fibrils [32]. The presence
of ACTH fragments in the culture of serotoninergic neu-
rons elicited the maturation of these cells and resulted in
a larger amount of nerve cell projections [33]. Introduc-
tion of ACTH4-10 and ACTH1-2 4 into a culture of rat em-
bryo brain cells yielded a larger density of neuronal nets
and neuron links [34]. It was also demonstrated that pe-
ripheral nerve regeneration induced by ACTH and its
ACTH4-10 fragment was accompanied by considerable
enhancement of the synthesis of RNA and proteins in
neurons. The fact that ACTH strengthened the synthesis
of RNA and proteins and glucose utilization in the brain
of old animals was also mentioned [35-37].
Regardless of the large body of experimental evidence
testifying to the extrahormonal activity of ACTH and its
fragments, it remains unclear to date which of the recap-
tors is involved in its nootropic, neuroprotective, and
neurotrophic actions, with the melanocortin receptors
MC3 and MC4 being the most probable candidates, as
they are expressed in the brain [38,39].
The discovery of the marked nootropic, neuroprotec-
tive, and neurotrophic effects of ACTH and its fragments
rendered this peptide family one of the most promising
candidates for the prevention and treatment of patholo-
gies of the central nervous system. However, the applica-
tion of these peptides in clinical practice appears to be
difficult because of, first and foremost, the extreme in-
stability and short-term effects of these peptides. There-
fore, numerous synthetic analogs, ORG 2766 [40-44],
HOE 427 [45,46], ORG 31433 [47,48], and BIM-22015
[49], which are more resistant to the action of proteases
and many times more active, were created by replacing
the natural left-handed amino acids in the ACTH mole-
cule with right-handed amino acid residues. Nevertheless,
the presence of nonnatural amino acids and radicals in
the structure of these ACTH analogs yielded some new
and at times negative biologic features; thus, no medical
preparations based on the ACTH analogs described
above have been produced.
In the late 1970s, a research team headed by Drs N.F.
Myasoedov and I.P. Ashmarin initiated studies to create
a nootropic peptide preparation based on ACTH and its
fragments. As stated earlier, the shortness of the effect of
natural ACTH fragments was a limitation to their appli-
cation, as the duration of action of the most effective
natural fragment, ACTH4-10, was as short as 30 - 60 min,
and an increase in the dose of the preparation adminis-
tered yielded no effect [10,11]. The study of the noot-
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A New Generation of Drugs: Synthetic Peptides Based on Natural Regulatory Peptides 227
ropic activity of some ACTH4-10 analogs with various
C-terminal region modifications showed that the analog
carrying a replacement of the three C-terminal amino
acids with the Pro-Gly-Pro sequence displayed the long-
est activity. This peptide (H-Met-Glu-His-Phe-Pro-Gly-
Pro-OH), termed Semax, exhibited the nootropic effect
of its natural prototype, but its effect lasted for 20 - 24 h.
The study of the hormonal action of the peptide revealed
that it had neither corticotropic nor melanocyte-stimu-
lating activities [50].
The medical preparation “Semax 0.1%” was produced
in the form of nasal drops, and successfully passed all
preclinical and clinical trials. It was introduced in clinical
practice as a neuroprotective and nootropic remedy. By
2001, a new form of the preparation, the “Semax 1%”
nasal drops, that was intended for therapy following
brain stroke was developed, and the range of the curative
and preventive effects of “Semax 1%” proved to be
broad, including effects on conditions such as brain
stroke, dyscirculatory encephalopathy, Parkinson’s dis-
ease, ocular nerve atrophy, and a number of chronic pa-
thologies caused by blood circulation deficiency in the
brain. Moreover, the high efficacy of “Semax 0.1%” in
preventive enhancement of intellectual capacity in heal-
thy persons in extreme stress conditions has been re-
ported. Finally, a pharmaceutical composition termed
“Minisem” was specifically developed for newborn chil-
dren (starting from the age of three) with neurological
defects.
Although a medical preparation based on Semax is
successfully used for prevention of and therapy for CNS
diseases, further studies on the range of its physiological
effects and mechanisms of action in the body are pres-
ently in progress.
3.2. Basic Biologic Properties of Semax
3.2.1. Neu roprotective Effect
Studies of the biologic effects of Semax in the body
showed that this peptide has pronounced neuroprotective
properties. It was found that Semax significantly reduced
the number of damaged cells under conditions of oxida-
tive stress caused by short-term incubation with hydro-
gen peroxide [51]. In addition, it significantly increased
the survival of cerebellar granule neurons during gluta-
mate neurotoxicity, which may be mediated by its effect
on calcium homeostasis and on the functional state of
mitochondria [52].
Experiments performed on cultures of embryonic rat
brain cells showed that Semax at a dose of 0.1 - 10 μM
increased the number of surviving neurons by 1.5 - 3.0
times compared with the control, without affecting the
proliferation of glial cells [53]. Subsequent studies re-
vealed that Semax at 100 nM increased the survival of
cholinergic neurons in the basal forebrain in vitro by 1.5
- 1.7 times and stimulated the activity of acetylcholine-
transferase [54].
Studies of the antihypoxic properties of Semax per-
formed on rats showed that this peptide increased the life
span of the animal 2.5-fold at an extreme altitude (12,000
m) and conferred adaptation to hypoxia. In studies per-
formed using volunteers, Kaplan et al. showed that Se-
max arrested the posthyperventilation effects of EEG
induced by a compensatory decrease in cerebral blood
flow [55].
Studies of the effect of Semax after acute hypobaric
hypoxia in rats of different ages showed that a single
Semax pretreatment (50 μg/kg) led to an increase in indi-
vidual resistance of animals to hypoxia, had a positive
effect on the parameters of cardiac function during hy-
poxia, and reduced the behavioral changes, which are
caused by oxygen deficiency [56]. A single intraperito-
neal injection of Semax (100 μg/kg) significantly in-
creased the time to breathing arrest in animals under hy-
poxia conditions. In addition, intranasal administration of
the peptide for 6 - 7 days at a dose of 100 μg/kg in-
creased the time to loss of posture and time to breathing
arrest [10,57].
Studies have shown that intraperitoneal injection of
Semax (100 μg/kg) significantly increased the overall
resistance of rats to circulatory hypoxia and the survival
of animals, and reduced the severity of the neurological
defect. Yasnetsov showed that prior curative administra-
tion of Semax significantly reduced neurological damage
in rats with cerebral ischemia caused by gravitational
overload. In this case, there was a reduction in the effect
of Semax with at high doses (in the range of 300 - 1200
μg/kg per day) [58].
The effect of Semax in global cerebral ischemia in rats
with reduced levels of cerebral blood flow was studied. It
was shown that the introduction of the peptide had a
positive effect on the dynamics of cerebral blood flow
and survival only in animals with a decompensated se-
vere hemodynamic disorder of the brain [59]. It was es-
tablished that double Semax intraperitoneal injection
(300 μg/kg) exerted a neuroprotective effect and almost
completely prevented the appearance of signs of neuro-
logical deficit and the increase in NO generation in the
brain caused by ischemia [60,61].
The neuroprotective and antiamnestic effects of Semax
have been demonstrated in experimental ischemic infarc-
tion in prefrontal cortical areas. Systemic Semax admini-
stration (250 μg/kg) after local ischemic infarction sig-
nificantly improved the rates of conditioned passive
avoidance reflex [62]. Further studies showed that in rats
with ischemic damage in prefrontal cortical areas,
chronic administration of Semax (250 μg/kg) for 6 days
after injury contributed to the restoration of spatial mem-
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A New Generation of Drugs: Synthetic Peptides Based on Natural Regulatory Peptides
228
ory and spatial learning ability [63].
The possibility of using Semax to correct postresusci-
tation neurological disorders was studied using a rat
model of clinical death. Ligation of the vascular bundle
of the heart was carried out to stop the general blood-
stream. Animals were revived using a closed cardiac
massage and artificial ventilation 10 min after clinical
death, followed by the assessment of the degree of mem-
ory of the experimental environment via testing in the
“open field”. Daily intranasal Semax administration (50
μg/kg) for 2 weeks normalized the parameters of orient-
ing-exploratory reaction in rats, bringing their values to
the levels of those of the control [64,65]. Clinical studies
of the effect of Semax on the restoration of amnestic
brain function in patients with severe postresuscitational
pathology (intellectual and mental disorders) showed that
the majority of patients treated with Semax exhibited a
significant improvement. A neuropsychological study
revealed an improvement in attention, memory, cognition,
mobility, muscle function, and audioverbal memory after
Semax treatment. Electroencephalography confirmed the
efficacy of Semax, as an improvement in the functional
state of the brain was noted [66].
To identify a possible mechanism underlying the neu-
roprotective action of Semax, attempts were made at
identifying the receptor of Semax using preparations of
plasma membranes of neurons and glial cells and
whole-brain neurons from rats. It was shown that Semax
bound to the nuclear membranes of nerve cells of the
basal forebrain of rats, which was specific and reversible
[67].
The results of the study performed by Vyunova et al.
showed the presence of the same type of specific binding
sites on the membranes of the rat hippocampus and
cerebellum; the highest number of binding sites was de-
tected in the hippocampus. Specific binding to the mem-
branes of the basal ganglia and cerebral cortex was found.
Moreover, specific binding of Semax prevented any pre-
mature hydrolysis, which apparently confirmed the long-
term pharmacological effect of the peptide [68].
3.2.2. Neuro trophic Effect
It has been suggested that the neuroprotective effects of
Semax are associated with its effect on the expression of
neurotrophins [53]. An experiment performed using a
culture of glial cells resulted in rapid (within 30 min after
Semax injection at 100 μg/kg) induction of the transcript-
tion of the neurotrophin Bdnf and Ngf genes [69]. Fur-
thermore, in vivo experiments have shown that in the
hippocampus of healthy rats, Semax increased the ex-
pression of neurotrophin genes at the protein (Bdnf and
TrkB) and mRNA (Bdnf, TrkB, and Ngf) levels [70-72].
Additional studies showed that a single Semax intra-
nasal administration (50 μg/kg) led to an increase in Bdnf
gene expression at the protein level in the rat hippocam-
pus (by 1.4 times), which was accompanied by increased
levels of tyrosine phosphorylation of TrkB (by 1.6 times)
[71].
Studies of the functional morphology and proliferative
activity of the brain cells of rats have shown that Semax
and its C-terminal fragment Pro-Gly-Pro activate the
capillary network and increase the proliferation of glia,
the endothelium of blood vessels, and progenitor cells in
the subventricular zone in healthy animals. A significant
increase in the expression of the nuclear protein PCNA,
which participates in the preparation for cell division,
was also noted. Judging by the upregulation of PCNA in
the nuclei of the brain cells of control animals, Semax
appears to stimulate the proliferation of repopulating
elements of the nervous tissue [73].
3.2.3. Nootr opic Effect
Studies of the effect of Semax on animal learning
showed that the intraperitoneal administration of the pep-
tide at doses of 15 - 150 μg/kg 5 min before and after the
training session accelerated the acquisition of a food-
getting habit in a T-maze. At similar doses of 15, 50, and
100 μg/kg administered 5 min before the training session,
Semax significantly accelerated the training of animals in
this test. The reduction of the dose to 5 μg/kg resulted in
the disappearance of the effect. Introduction of Semax at
a dose of 500 μg/kg did not influence learning by rats in
the test used. Consequently, the increase in Semax dose
does not lead to the reversal of its effects [10].
A positive effect of Semax on learning in animals re-
garding the conditioned passive avoidance reflex has
been demonstrated. The introduction of the peptide (15
μg/kg) both 1 and 6 h before a training session improved
learning by animals in this test. Consequently, the impact
of Semax on the conditioned passive avoidance reflex
does not depend on the sign of the reinforcing stimulus
[10]. Semax also stimulated the amnestic processes not
only in intact animals, but also in terms of pathology.
Injection of the peptide before electroshock to test the
conservation of passive avoidance in animals that suf-
fered an electric shock led to a partial restoration of skills
and attenuated amnesia caused by electroshock [11].
The study of the nootropic activity of Semax using
various manners of drug administration in rats showed
that the greatest nootropic effect of the peptide on the
conditioned passive avoidance reflex was observed after
intranasal administration of the peptide, whereas intrap-
eritoneal injection yielded less pronounced effects [74].
Concomitantly, the effects of Semax after a single intra-
nasal administration appeared after 4 min and lasted for
24 h, which is 50 times greater than the length of action
of the ACTH4–10 peptide [75,76]. In newborn rats, Semax
contributed to increased selective attention and research
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activity and reduced the level of anxiety [77,78].
The work of Kaplan et al. using human volunteers
showed that intranasal administration of the peptide (16
μg/kg) significantly increased the attention and short-
term memory of the subjects during testing, both at the
beginning and at the end of the day. The introduction of
the peptide at a dose of 250 - 1000 μg/kg caused changes
in electroencephalographic parameters that were similar
to the changes that arise after the administration of typi-
cal neuroprotective drugs [79].
Thus, these data seem to suggest that Semax can be
administered to healthy individuals to prevent the nega-
tive impact of the environment, in particular the impact
of long-term psychoemotional stress (increased informa-
tion load, the effect of intense muscular workload, pro-
fessional work in difficult conditions, a high degree of
responsibility, conflict, financial problems, etc.).
3.2.4. An xi olytic Ac tion
The c-fos gene is overexpressed in different brain struc-
tures during emotional stress. The most pronounced ex-
pression of this gene was detected in the brain of animals
that were susceptible to emotional stress. Preliminary
intraperitoneal Semax administration caused a decrease
in stress-induced c-fos gene expression in the paraven-
tricular hypothalamus of rats predisposed to emotional
stress [80].
Prior administration of the delta-sleep-inducing pep-
tide and Semax to rats in the normal state increased c-fos
gene expression, whereas the same oligopeptides inhib-
ited emotional-stress-induced early c-fos gene expression.
The change in the stress-induced expression pattern of
the c-fos gene after administration of the delta-sleep-
inducing peptide and Semax may be due to the fact that
the reception of these substances changes in these brain
structures under emotional stress [81].
A study has shown that chronic intranasal Semax ad-
ministration in white rats (50 μg/kg) for 10 - 14 days in-
duced an anxiolytic and antidepressant activity, but had
no effect on exploratory activity in nonstress conditions.
The authors of that work attribute this effect of Semax to
its ability to activate the serotoninergic system of the
brain [82].
The effect of a single intranasal administration of Se-
max (at doses of 50 and 500 μg/kg) on the levels of anxi-
ety and depression in white rats has been studied. Semax
had no effect on anxiety and depression in normal ani-
mals. However, when the levels of anxiety and depress-
sion were increased by the administration of the chole-
cystokinin-tetrapeptide, Semax had anxiolytic and anti-
depressant effects. The absence of anxiogenic activity
could be due to the lack of the Phe-Arg-Trp-Gly se-
quence in its structure, which is available in the natural
ACTH4-10 prototype and determines the anxiogenic prop-
erties of natural melanocortins [83].
A positive modulator effect of Semax on the sero-
toninergic system of the striatum was also shown [84],
with reduction of nociception [85] and normalization of
the circadian locomotor rhythms [86].
A single intraperitoneal Semax injection at doses of
150 and 600 μg/kg increased the levels of dopamine,
serotonin, and its metabolite 5-hydroxyindoleacetic acid.
This effect was observed within 24 h after the admini-
stration of the peptide. Chronic daily administration of
Semax (600 μg/kg per day) for 7 days tended to reduce
the levels of dopamine and serotonin significantly in the
hypothalamus. These findings suggest the accelerated
turnover of serotonin, reflecting an increase in the func-
tional activity of this mediatory system [87]. Pretreat-
ment with Semax potentiated the effects of D-ampheta-
mine on the dopaminergic system of the striatum and on
the locomotor activity of rats. The authors of that work
suggest that Semax increases dopamine transmission in
the striatum [88].
3.3. Pharmacokinetics
A comparative study of the stability of the Semax peptide
and that of the native ACTH4–10 to the action of proteases
in human blood serum showed that the primary degrada-
tion step in both cases is the cleavage of the N-terminal
methionine by aminopeptidase. However, the natural
fragment is rapidly degraded to individual amino acids,
whereas Semax forms rather stable intermediates with
the structure Glu-His-Phe-Pro-Gly-Pro, which exhibit
neurotrophic activity in the same range as that of Semax
[75,76]. Subsequently, this fragment undergoes sequen-
tial degradation under the action of dipeptidyl aminopep-
tidase, which cleaves it to His-Phe-Pro-Gly-Pro and Pro-
Gly-Pro. Determination of the levels of Pro-Gly-Pro in
the serum within 3 - 5 h of the introduction of Semax
using different approaches (intraperitoneal, intragastric,
and intracolonic) revealed that its content does not ex-
ceed 2% - 5% of the total amount of injected peptide.
The Pro-Gly-Pro peptide is hydrolyzed by enzymes of
the serum, forming fragments of Pro-Gly and Gly-Pro,
and influences the number of processes in the body.
Subsequently, aminopeptidase and the angiotensin-con-
verting enzyme degrade these fragments to amino acids
[89]. The half-life of Semax in the serum is >1 h. Its
prolonged effect is associated with the cytoprotective
activity of its fragments [90].
The study of Semax degradation in the presence of
plasma membranes of nerve cells, as well as in cultures
of glial and nerve cells derived from rat forebrain,
showed that Semax degradation retains the N- and C-
termini in all cases. However, the cleavage of the peptide
containing the N terminus is more intense. The main
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route of degradation is the cleavage of the C- and N-
terminal dipeptides, to form the pentapeptides His-Phe-
Pro-Gly-Pro and Met-Glu-His-Phe-Pro [91]. Thus, as a
result of fairly rapid enzymatic degradation, Semax in
brain tissues is a mixture of peptide derivatives with dif-
ferent compositions [92].
A comparative study of the nootropic activity of the
products of the enzymatic degradation of ACTH4-10 and
Semax (Glu-His-Phe-Pro-Gly-Pro, His-Phe-Pro-Gly-Pro,
and Phe-Pro-Gly-Pro) showed that ACTH4-5 and Glu-
His-Phe-Pro-Gly-Pro have neurotropic activity [93].
The results of that study led to the conclusion that the
products of the enzymatic degradation of ACTH4–10 and
Semax have their own neurotrophic activity and are syn-
ergistic regarding their effect on learning processes.
Consequently, the observed effects of these peptides may
be the result of the global action of the original heptapep-
tide and its derivatives. The high activity and relative
stability of the hexapeptide Glu-His-Phe-Pro-Gly-Pro
greatly ensures the long-term effect of Semax [11].
After intravenous administration, about 0.01% of Se-
max penetrates the blood–brain barrier [76,94], whereas
0.093% of Semax penetrates this barrier 2 min after in-
tranasal administration [92]. Semax has a half-life in the
body of a few minutes, and its therapeutic effect persists
for 20 - 24 h after the administration of a single dose [76].
Consequently, intranasal Semax administration is as ef-
fective as intraperitoneal injection. This is the basis for
the recommendation of the intranasal route for Semax
administration in humans.
3.4. Effect of Semax on Gene Transcription
One important aspect in the study of the mechanism of
action of natural regulatory peptides and their synthetic
analogs is their ability to influence gene expression at
both the mRNA and protein levels. The ability of Semax
to increase the survival of rat brain neurons and the exis-
tence of Semax-binding sites on brain plasma membranes
indicate that the physiological effects of the peptide can
be exerted through the influence of the state or function
of brain cells. Grivennikov et al. [53] suggested that the
neuroprotective effects of Semax are associated with its
effect on the expression of a family of regulatory proteins,
the nervous-tissue neurotrophins, which are synthesized
by neurons and glial cells and promote the differentiation
and the maintenance of the viability and function of pe-
ripheral and central neurons.
Shadrina et al. [69] first showed that Semax caused a
rapid (within 30 min) induction of the transcription of the
neurotrophin Bdnf and Ngf genes in glial cell cultures.
The Bdnf mRNA levels increased 8-fold and the Ngf
mRNA levels increased 5-fold. The results of subsequent
in vivo experiments demonstrated that Semax upregulates
neurotrophin genes (Bdnf, TrkB, and Ngf) at the mRNA
level in the hippocampus of healthy rats [70]. Further
studies showed that a single intranasal administration of
Semax (50 μg/kg) led to upregulation of the Bdnf and
TrkB mRNAs in the rat hippocampus by 3- and 2-fold,
respectively [71].
Agapova et al. [72] assessed the effect of the admini-
stration of a single intranasal dose of Semax (50 μg/kg)
on the maintenance of Bdnf and Ngf mRNA levels in
different parts of the rat brain and retina. One hour after
Semax administration, a significant increase in Bdnf gene
expression was observed in the hippocampus, cerebellum,
and brain stem. The levels of Ngf mRNA were signifi-
cantly increased in the hippocampus and decreased in the
frontal cortex.
Evaluation of the temporal dynamics of the changes in
neurotrophin mRNA levels showed that the intranasal
administration of a single dose of Semax (50 μg/kg)
yielded a rapid response of the Bdnf and Ngf genes in the
rat hippocampus and frontal cortex, as demonstrated by a
significant change in their expression within 20 min after
Semax administration. A decrease in Bdnf and Ngf
mRNA levels was observed in the hippocampus. In con-
trast, an increase in the mRNA levels of these genes was
observed in the frontal cortex. A similar temporal dy-
namics of the changes in the expression of these two
genes was observed within the same brain structures. In
addition, two peaks of expression were noted for both
genes: the first peak was observed 1.5 h after administra-
tion of Semax (Bdnf, 1.38 and 1.32 times in the hippo-
campus and frontal cortex, respectively; Ngf, 1.38 and
1.11 times, respectively) and the second peak was ob-
served 8 h after administration of Semax (Bdnf, 1.75 and
1.64 times in the hippocampus and frontal cortex, respec-
tively; Ngf, 1.67 and 1.65 times, respectively) [95].
Later, it was shown that the intranasal administration
of a single dose of Semax at a similar concentration elic-
ited both fast and long-term changes in Bdnf and Ngf
gene expression in the rat hippocampus, frontal cortex,
and the retina. It was noted that the temporal dynamics of
the changes in the expression of these genes under the
influence of Semax in all tissues examined was similar.
The greatest increase in the mRNA levels of both genes
was observed in the hippocampus, frontal cortex, and
retina 8 h after the administration of the peptide [96].
Agapova et al. studied the effect of Semax on the ex-
pression of genes involved in intracellular signaling
pathways in the hippocampus of healthy animals using
the “RT²Profiler™ PCR Array Signal Transduction
Pathway Finder” panel (SABioscience, USA), which
includes 84 genes. Those authors found that the intrana-
sal administration of a single dose of the peptide (50
μg/kg) led to a change in the mRNA levels of several
genes involved in the implementation of the Wnt (Ccnd1,
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Cdh1, Pparg, and Vegf) and NF-kB (Birc2, Tnfb, If1,
Nos2a/iNos, and Tnfα) pathways, which are involved in
the regulation of processes associated with the survival
of neurons and neuronal cell cultures [97].
To elucidate the molecular mechanism underlying the
neuroprotective actions of Semax in pathological condi-
tions, the effect of Semax and its most stable metabolite
(Pro-Gly-Pro) on the mRNA content of genes encoding
growth factors and their receptors was investigated in
focal cerebral ischemia. For this purpose, the “RT²Pro-
filer™ PCR Array Rat Neurotrophin and Receptors”
panel (SABioscience, USA) was used. Among the 84
genes studied, the expression of 19 genes was signifi-
cantly altered. During ischemia, Semax and its Pro-Gly-
Pro metabolite had an impact on the expression of
mRNA growth factors and their receptors, increasing the
expression of some and reducing that of others. The
spectrum of genes that exhibited a change in expression
levels under the influence of both peptides overlaps only
partially. The most pronounced changes (by 2 or more
times) in expression under the influence of Semax and
Pro-Gly-Pro were observed for Lif, Galr, TrkA, and p75.
Thus, these studies have shown that the mechanisms of
action of Semax and Pro-Gly-Pro on the nervous tissue
exhibit specific features. The authors suggested the exis-
tence of two types of growth-factor targets: neural tissues
(neurons and glial cells) and cells of the vascular wall
[98].
Subsequently, those authors investigated the effect of
Semax and Pro-Gly-Pro on the transcription of neuro-
trophins and their receptors in the cortex of rats with fo-
cal cerebral ischemia. Semax increased the transcription
of Bdnf, TrkC, and TrkA at 3 h, Nt-3 and Ngf at 24 h, and
Ngf at 72 h after occlusion. Pro-Gly-Pro, in turn, in-
creased the transcription of Bdnf and TrkC at 3 h and Ngf,
TrkB, TrkC, and TrkA at 24 h after occlusion. Hence, it
was suggested that the activation of the expression of the
neurotrophin system under Semax and Pro-Gly-Pro ac-
tion may contribute to the neuroprotection and survival
of neural cells after ischemia. It should be noted that this
activating effect was mainly observed 3 h and 24 h after
the ischemic attack, a time at which some cells in the
peripheral ischemic focus retain functional activity and
are able to survive. In addition, Semax administration led
to changes in the expression of neurotrophins and their
receptors mainly in the ischemic cortex, whereas the ef-
fect of Pro-Gly-Pro was less specific. This may indicate
that these peptides have specific mechanisms of action
that differ from each other [99].
Stavchansky et al. studied the effect of the synthetic
peptide Semax and its C-terminal fragment Pro-Gly-Pro
on the expression of neurotrophin genes (Ngf, Bdnf, and
Nt-3) and their receptors (TrkA, TrkB, TrkC, and p75) in
the frontal lobes, hippocampus, and cerebellum of a rat
model of incomplete global cerebral ischemia. A change
in the expression of many genes (in particular those en-
coding neurotrophin receptors) has been observed in the
frontal cortex of ischemic animals. The most noticeable
ischemic effect acted on the expression of neurotrophins
and their receptors in the hippocampus. A reduction in
the relative content of the mRNA levels of Nt-3, TrkB,
TrkC, p75, and TrkA genes was observed within the first
few hours after occlusion, and a significant decrease in
the expression of most genes studied was observed 8 h
and 12 h after occlusion.
Thus, the administration of Semax and Pro-Gly-Pro
has a significant effect on the expression of neurotrophin
genes and their receptors predominantly in the frontal
cortex and hippocampus of ischemic animals. In the
frontal cortex of rats under the influence of Semax, the
mRNA levels of neurotrophin receptor genes were
downregulated, whereas they were upregulated under the
influence of Pro-Gly-Pro. The maximal neuroprotective
effect of peptides was observed in the hippocampus of
animals 12 h after occlusion, a time at which the ische-
mic injury resulted in a significant decrease in the ex-
pression of neurotrophin genes and their receptors. Thus,
the use of Semax and Pro-Gly-Pro at this time offsets
ischemia-mediated changes in gene expression [100].
3.5. Clinical Application
3.5.1. Neurology
1) Vascular Brain Diseases
Vascular diseases of the brain are a major cause of
death worldwide with ischemic brain lesions being pre-
dominant (80%) among the cerebrovascular diseases. In
recent years, a clear temporal sequence of molecular and
biochemical mechanisms that trigger acute focal cerebral
ischemia has been established. The close relationship
among the long-term effects of ischemia as well as their
common trigger mechanisms allow for secondary neuro-
protection in addition to local action via the use of the
modulating effect of regulators, including the major role
played by neuropeptides [101-105]. Given that neuro-
peptides can easily penetrate the blood-brain barrier (in
contrast with the polypeptide chains of growth factors); it
is difficult to overestimate their potential therapeutic sig-
nificance.
Experiments using tissue cultures and animal models
of cerebral ischemia, as well as clinical conditions, estab-
lished the neuroprotective effect of the drug “Semax 1%”,
the main effects of which are the inhibition of glial in-
flammatory responses, improvement of the trophic sup-
port of the brain, inhibition of nitric oxide synthesis, and
other reactions of oxidative stress and immunomodula-
tion. The steps in the Semax-induced chain of metabolic
transformations reinforce and support each other, leading
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to inhibition of the most important mechanisms of de-
layed cell death [106].
The evaluation of the effectiveness of “Semax 1%” in
the treatment of patients with various cerebrovascular
diseases showed that after treatment, most patients ex-
hibited a significant improvement in overall health (re-
duced headaches, normal sleep, increased efficiency,
increased level of memory and concentration, reduced
anxiety) [107].
Clinical studies showed a positive effect of Semax in
acute hemispheric ischemic stroke. Intranasal Semax
administration as an intensive therapy for acute hemi-
spheric ischemic stroke had a beneficial effect on the
quality and pace of the recovery processes, as it helped to
accelerate the regression of brain and focal abnormalities
[108].
The role of immune responses and local inflammation
in the pathogenesis of ischemic stroke and in the forma-
tion of infarction changes in brain tissues is well docu-
mented. Their importance in the development of an in-
flammatory response not only to the excessive release of
proinflammatory cytokines (IL-1β, TNF-α, and IL-8), but
also to the lack of protective anti-inflammatory and tro-
photropic factors (IL-10 and TGF-β) has been established
[106,108,109]. The investigation of the immunobio-
chemical mechanisms underlying the neuroprotective
action of Semax in acute ischemic stroke showed that the
introduction of the peptide leads to a change in the bal-
ance of peptidergic systems of the brain toward the pre-
dominance of anti-inflammatory and trophic protective
factors (IL-10, TNF-α, and TRF-β) over proinflammatory
factors (IL-8 and C-reactive protein) [110].
Thus, those authors showed that the drug “Semax 1%”
is well tolerated and has no toxicity or significant side
effects (indicated by a discoloration of the mucous mem-
brane of the nasal cavity in 10% of patients and a mild
increase in the concentration of glucose in the blood of
7.4% of patients with diabetes). Therefore, Semax has a
strong neuroprotective effect. The Semax-induced chain
of metabolic reactions plays a role in all of the major
mechanisms underlying the long-term effects of ischemia,
underscoring the promise of its application as a neuro-
protector in patients with ischemic stroke.
2) Dyscirculatory Encephalopathy
Dyscirculatory encephalopathy is the progressive chro-
nic cerebrovascular insufficiency that leads to the devel-
opment of multiple small focal necroses of brain tissue
and manifests itself as gradually increasing defects in
brain function [111,112].
A clinical trial of the action of Semax in patients at
different stages of dyscirculatory encephalopathy showed
that Semax treatment (9 and 12 mg per day) led to sig-
nificant clinical improvement, helped stabilize the pro-
gression of the disease, and reduced the risk of stroke and
transient ischemic attacks in the course of the disease.
The drug elicited a small percentage of side effects,
which were well tolerated by patients (including older
patients) [113].
3.5.2. Ophthalmolo g y
1) Optic Nerve Atrophy
Clinical studies of the efficacy of “Semax 0.1%” as a
therapy for brain diseases were a prerequisite for clinical
efficacy studies on optic nerve disease, which is one of
the worst pathologies of the visual analyzer and often
leads to partial or complete atrophy of the optic nerve
and, consequently, impaired vision or blindness [114].
Studies have shown that the inclusion of “Semax
0.1%” in complex therapy for optic nerve diseases has a
beneficial effect on the quality and pace of the recovery
processes and helps improve visual function. The result
is the directly stimulating effect of an electric current on
the activation of blood circulation, which increases cell
metabolism activity [115].
The use of “Semax 0.1%”, particularly in the acute
stage of diseases of the optic nerve, effectively protects
nervous tissues from the effects of damage by signifi-
cantly increasing the positive clinical dynamics of the
estimated increase in visual acuity, expanding the total
field of view, increasing the electrical conductivity and
the sensitivity of the optic nerve, and improving color
vision [116].
2) Glaucoma Optic Neuropathy
According to modern concepts, the pathogenesis of
glaucoma optic neuropathy, which is characterized by
loss of retinal ganglion cells, has much in common with
that of chronic cerebral ischemia [102,117,118]. The
considerable successes in the treatment of ischemic neu-
rological diseases, as well as of some forms of optic at-
rophy, with “Semax 0.1%” suggest its potential thera-
peutic effect in the treatment of glaucoma optic neuropa-
thy [110,116].
Research on the effectiveness of “Semax 0.1%” in the
treatment of primary open-angle glaucoma at the devel-
oped and advanced stages in patients with normalized
ophthalmotonus showed that 1 month after initiation of
treatment, a number of functional indicators that reflect
the status of the optic nerve (relative and absolute num-
ber of defects in the central field of view and color sensi-
tivity) were significantly improved compared with those
of patients who received only basic therapy.
The best results of treatment on all indicators were
achieved in patients with advanced glaucoma and ad-
vanced glaucoma compensated by surgery who received
“Semax 0.1%” via electrophoresis for 10 days, followed
by intranasal instillation within 10 days. These patients
exhibited significantly improved visual acuity, and ex-
tension of the boundaries of the visual fields [115].
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3) Nonproliferative Diabetic Neuropathy
We evaluated the efficacy of the drug “Semax 0.1%”
in the treatment of patients with nonproliferative diabetic
retinopathy, which develops in the presence of diabetes
mellitus type II. All patients exhibited positive dynamics
of the functional parameters (perimetric and electro-
physiological) after the treatment [119].
4. Selank Is a New-Generation Antianxiety
and Noot-Ropic Compound
4.1. Background
A common system of regulation exists among the major
regulatory systems of the body. Based on this fact, it has
been suggested that endogenous regulators of one system
can affect the regulation of another [120].
Tuftsin was one of the first peptides of the immune
system for which a distinct influence on the functions of
the CNS was described. Tuftsin is a tetrapeptide with the
sequence Thr-Lys-Pro-Arg. It was isolated in 1970 by
Najjar and Nishioka from the leucophilic fraction of the
IgG protein [121]. Tuftsin is a 289 - 292 amino acid
stretch of the CH2 domain of the Fc fragment of the leu-
cokinin molecule [122,123].
Tuftsin acquires its activity after excision from the
protein carrier via the successive action of two enzymes:
splenic tuftsin endocarboxypeptidase, which cuts the C
terminus of tuftsin, and the phagocytic enzyme leu-
cokinase, which cuts its N terminus [124-127].
Specific receptors for tuftsin are found exclusively in
cells that have Fc receptors for IgG: monocytes/macro-
phages, neutrophils, and NK cells. After tuftsin-receptor
interaction, the complex is internalized by macrophages
[128]. In vivo, tuftsin is delivered to cells that possess
specific receptors as part of the “antigen-antibody” com-
plex [129].
The principal biologic activity of tuftsin consists of the
activation of phagocytosis by granulocytes and macro-
phages. It also activates pinocytosis, increases the respi-
ratory burst of phagocytic cells (thus stimulating their
bactericidal activity), destroys neoplastic cells, and af-
fects the formation of antibodies. Tuftsin stimulates the
formation of superoxide and nitroxide radicals by macro-
phages, which leads to an increase in their digestive ca-
pacity [124,130,131]. In vivo experiments showed that
tuftsin induces strong antibacterial activity without ap-
parent toxicity. It increases the cytotoxic action of T
lymphocytes and stimulates the synthesis of antibodies
[132-134].
In the early 1980s, Valdman and Ashmarin hypothe-
sized that the regulator of the peripheral immune system,
tuftsin (which exhibited structural similarity to a family
of neuropeptides), has an impact on the CNS [9,135].
Herman et al. found that the intraventricular administra-
tion of tuftsin to rats elicited analgesia that lasted for 20
min, as assessed using the hot-plate test [136,137].
Among the various tuftsin analogs only the dipeptide
Pro-Arg exhibited any evident analgesic action, as as-
sessed using both the hot-plate and the tail-immersion
tests [138,139].
Intraperitoneal administration of tuftsin at a dose of
500 mg/kg yielded enhanced locomotor activity and ag-
gressiveness and diminution of the acquisition of the
passive avoidance reaction during a single reinforcement
[135]. Semenova et al. found that IP injection (300 mg/
kg) of tuftsin to rats induced an increase in the stability
of memory traces during a 30-day period and affected
learning and exploratory behavior in rats [140,141].
Experiments performed using dogs, cats, rabbits, and
rats showed that tuftsin modifies the configuration of the
evoked potentials, especially in the visual cortex, as well
as augmenting catecholaminergic and suppressing sero-
toninergic activity in the sensorimotor cortex and caudate
nucleus [142,143]. Administration of a single dose of
tuftsin distinctly altered the synthesis and degradation of
monoamines, acetylcholine, and proteins in the cortex-
subcortex structures of the brain locomotor system. Tuft-
sin normalizes the levels of dopamine, norepinephrine,
and serotonin [144].
The relation between the brain monoaminergic sys-
tems and the effects of tuftsin on animal emotional be-
havior was also studied by Semenova et al. Those au-
thors found that intracutaneous administration of tuftsin
to rats neonatally treated with 5,7-dihydroxytryptamine
resulted in a weakened perception of stress situations, an
increase in the stability of investigative behavior, and a
normalization of serotonin levels in the brain [145].
This wide biologic activity of tuftsin is of considerable
interest in relation to the use of this peptide and its de-
rivatives as drugs. However, natural peptides are highly
unstable when injected into the body, and have a short-
term effect. Russian scientists conducted a detailed study
of the synthesis and long-acting effect of tuftsin analogs.
The result was а heptapeptide containing the sequence
Thr-Lys-Pro-Arg elongated with the C-terminal part of
the tripeptide Pro-Gly-Pro, which proved to be most
suitable for the stabilization of the molecule and en-
hanced its resistance to proteases. The resulting peptide,
H-Thr-Lys-Pro-Arg-Pro-Gly-Pro-OH, which was named
Selank, had a pronounced anxiolytic activity. The drug
“Selank 0.15%”, in the form of nasal drops, was devel-
oped based on this peptide and has successfully passed
all preclinical and clinical trials. In 2009, the drug was
registered by the Russian Federation Ministry of Health
and approved for medical use. It has been successfully
introduced into clinical practice as an anxiolytic and
nootropic drug and has been widely recognized by spe-
cialists. However, despite the successful use of “Selank
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0.15%” in the prevention and treatment of generalized
anxiety disorder and neurasthenia, the study of the spec-
trum of its physiological effects and mechanisms of ac-
tion is still ongoing.
4.2. Basic Biologic Properties of Selank
4.2.1. An xiolyti c Ef fect
In 1960, Sudakov hypothesized that the resistance of
biologic objects to emotional stress should be determined
by endogenous peptide substances [146]. Studies aimed
at exploring the psychotropic and anxiolytic properties of
peptide compounds revealed that the most active in this
regard were drugs belonging to the tuftsin family [135,
147].
The study of the adaptive behavior of animals in stress
situations under treatment with tuftsin and its derivatives
showed that the most universally positive effect of this
drug in all model situations included Selank and the pen-
tapeptide Pro-Arg-Pro-Gly-Pro (intraperitoneal injection
at a dose of 300 μg/kg). Its peptides exhibited the most
pronounced anxiolytic effect and significantly activated
the exploratory activity of animals [147,148].
Sollertinskaya et al. showed that Selank has pro-
nounced neuropsychotropic, antidepressant, and antistress
effects and abolishes the reaction of aggression and fear
in primates with neurosis. The compensatory effects of
Selank have a dose-dependent character. The dynamics
of recovery of psychic functions in previously neurotized
animals after use of the preparation at high doses (300
μg/kg) is longer than that observed after the use of low
doses (30 - 50 μg/kg). The compensatory effects of Se-
lank occur in both inhibitory and excitatory neurosis
types. Compared with the neurohormones studied previ-
ously (thyroliberin, vasopressin, and ACTH4-10), Selank
has a prolonged effect and no side effects and, on the
whole, is unidirected in young and elderly monkeys
[149].
Seredenin et al. studied the behavioral effects of Se-
lank in BALB/c and C57BL/6 mice with different types
of emotional stress reactions. It was shown that the ad-
ministration of Selank at doses of 200 - 3000 μg/kg under
avoidable and unavoidable stress had a significant anx-
iolytic effect on BALB/c mice with an initially high level
of anxiety and had no significant effect on C57BL/6 mice
with low levels of anxiety and an active type of emo-
tional stress reaction. This effect of Selank is comparable
to the effect of benzodiazepine tranquilizers at low doses;
however, the action of Selank is not accompanied by
undesirable characteristics (hypnosedation and myore-
laxation) and side effects (amnesia, withdrawal, and de-
pendence) [150,151].
Narkevich et al. have shown that Selank administra-
tion to mice with different types of emotional stress reac-
tions in the “open field” test causes significant variations
in the levels of noradrenaline (NA), dopamine (DA),
serotonin (5-HT), and their major metabolites in different
brain regions. In particular, the content of 5-HT and its
metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the
hippocampus of BALB/c mice with an initially high level
of anxiety was higher than that of C57BL/6 animals with
an active type of emotional stress reaction. Selank ad-
ministration (300 μg/kg) caused an increase in NA con-
tent in the hypothalamus of both lines of mice and
changes in concentrations of DA metabolites (3,4-dihy-
droxyphenylacetic and homovanillic acids) in the hippo-
campus and frontal cortex. C57BL/6 mice exhibited a
significant increase in the value of these indicators,
whereas BALB/с mice showed a decrease in these values.
The administration of Selank caused a significant de-
crease in serotonin and 5-HIAA concentration in the
hippocampus only of BALB/c mice. These results con-
firmed the selectivity of the Selank effects [152].
The anxiolytic action of Selank under Naloxone-in-
duced disturbance of the brain opioid system has been
studied. It was shown that intraperitoneal Selank admini-
stration (250 μg/kg) caused an increase in the general
motor activity of BALB/c mice and had no effect on the
behavior of C57BL/6 mice. Naloxone altered the phenol-
type of the source of emotional and stress responses in
BALB/с mice, causing a reaction of “rapid escape” in the
“open field” peripheral zone, and eliciting the opposite
effect in C57BL/6 mice, as reflected in the fading reac-
tion. After Naloxone treatment, the sensitivity to the
anxiolytic action of Selank decreased in BALB/с mice
and increased in C57BL/6 mice. These results allowed
the identification of a new target of Selank action in the
CNS and suggested the importance of the activity of
opioid systems in the formation of individual sensitivity
to Selank [153].
The anxiolytic effect of Selank was studied in a new
model of innate (genetically determined) behavioral
symptoms of depression, the rat inbred line WAG/Rij. It
was found that repeated Selank administration in large
doses (1000 - 2000 μg/kg) eliminated the symptoms of
depression in the behavior of rats, but had no effect on
the level of general motor activity. Thus, these results
demonstrated the presence of a specific component in the
spectrum of Selank antidepressant activity [154].
The mechanism underlying the action of Selank re-
mains unclear; however, it is assumed that it is based on
the ability of the peptide to inhibit enkephalin-degrading
enzymes and to prolong the half-life (τ1/2) of opioids in
the body [155]. It is known that opioid peptides are in-
volved in the pathogenesis of anxiety and phobic states.
Thus, animals with knockout of δ-opioid receptors ex-
hibit elevated levels of anxiety [156]. Behavioral ex-
periments revealed the anxiolytic effects of an agonist of
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A New Generation of Drugs: Synthetic Peptides Based on Natural Regulatory Peptides 235
δ-opioid receptors, dalargin [157].
Intraperitoneal administration of Selank at doses of 10,
100, 1000, and 10,000 μg/kg reduced the behavioral
manifestations of apomorphine-induced dopaminergic
system hyperactivity and completely blocked the effects
of the nonselective opioid receptor antagonist Naloxone.
However, the direct binding of the peptide to dopamine
and opioid receptors has been reported. As enkephalinase
inhibitors together with opioid receptor agonists and an-
tagonists modulate the status of the dopaminergic system,
the authors assumed that the effect of Selank is mediated
by its ability to modulate the status of the endogenous
opioid system via changes in the activity of enkephalin-
degrading enzymes [158].
A radioligand analysis indicated that the τ1/2 of en-
kephalin in the blood of mice varies according to the
different types of emotional stress response. In intact
BALB/c mice with high levels of anxiety, this character-
istic is much lower than that observed in stress-resistant
C57BL/6 mice. This ratio was maintained in stressful
conditions. This result confirms the selective anxiolytic
effects of Selank in animals with an initially higher level
of enkephalinase activity. Thus, Selank increases the
functional activity of the endogenous opioid system,
thereby prolonging the circulation time of enkephalins in
the blood [159].
In addition, peptides generally labeled with tritium
were used to show that Selank significantly decelerates
the degradation of enkephalin by human blood plasma
enzymes. Selank completely inhibits carboxypeptidases,
decreases the activity of dipeptidylcarboxypeptidases
almost 20-fold, and reduces the accumulation of the
products of the N-terminal hydrolysis of Leu-enkephalin
only 2-fold. Consequently, Selank can be considered a
relatively selective inhibitor of carboxy- and dipeptidyl-
carboxypeptidases of blood plasma, which cleave Leu-
enkephalin. In the presence of 15 μM Selank, the bio-
degradation of Leu-enkephalin in blood plasma proceeds
in a more selective manner compared with a condition
without the drug, via a pathway associated with the ac-
tion of aminopeptidases. These enzymes hydrolyze not
only enkephalins, but also several other regulatory pep-
tides. Thus, the mechanism underlying the biologic ac-
tivity of Selank may be associated with its effect not only
on the opioid system, but also on several other systems of
regulatory peptides [160].
The effect of tuftsin and Selank on serotonin exchange
in the rat brain after pretreatment for 4 days with the se-
rotonin-synthesis inhibitor p-chlorophenylalanine (PCPA)
was studied. Increased metabolism of serotonin was ob-
served 30 min after Selank injection into the caudal sec-
tion of the brainstem—the area of localization of the
dorsal raphe nuclei, in which synthesis of serotonin oc-
curs. In contrast, the metabolic rate remained unchanged
in animals injected with PCPA after tuftsin administra-
tion [161].
4.2.2. Nootr opic Effect
Tuftsin and its derivatives have an effect on learning and
memory in rats. It has been established that Selank has a
significant positive action on learning processes in rats
via a conditioned passive avoidance reflex [162]. The
investigation of the conditioned active avoidance reflex
in rats with different phenotypes regarding emotional and
stress reactions (active and passive) revealed that Selank
(300 μg/kg) significantly enhances the learning process
in both groups. The highest Selank efficiency was
achieved on day 3 of training, at the stage of the outbreak
and emerging memory consolidation, and progressively
increased with further administration of the peptide. A
significant increase in the number of correct responses
was attributed to the antianxiolytic component of Selank.
It is known that anxiety and fear hamper the process of
conditioned reflex reactions to aversive effects. An acti-
vating effect of Selank was observed after its administra-
tion to rats with initially low levels of learning. Under
these conditions, this activating effect was observed on
the first day after the single injection of the peptide and
increased progressively with repeated administration
over the entire period of study [163,164].
There is a relationship between stressors and diseases
of the CNS, such as anxiety and depression [165]. In
addition, the pathophysiological effects of stress are as-
sociated with exposure to the functions of the hippo-
campus, which may be regulated by the brain-derived
neurotrophic factor (BDNF). BDNF is involved in learn-
ing and the formation of memory engrams in the mam-
malian brain and regulates synaptic function of short-
term and long-term synaptic potentiating by binding to
specific TrkB receptors on postsynaptic neurons [166,
167]. The downregulation of endogenous BDNF in rat
brain leads to disturbances in learning and memory for-
mation [168]. The investigation of the effect of Selank on
Bdnf gene expression in the hippocampus of rats showed
that the peptide increases the levels of the Bdnf mRNA 3
h after its administration (250 and 500 μg/kg) and the
levels of the BDNF protein 24 h after its administration
at both doses. Increased expression of BDNF suggests
that Selank stimulates the expression of the neurotrophic
factor in cells of the hippocampus, but not its axonal
transport from remote areas of the brain [169].
The ability of Selank to adjust the parameters of inte-
grative brain activity and the level of endogenous amines
in adult rats subjected to antenatal hypoxia has been de-
scribed. Hypoxia disrupts the balance of natural amines
in brain structures, emotional behavior, and cognitive
processes. These rats manifest significant alterations in
cognitive function, emotional behavior, and the balance
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of activity in monoaminergic systems. These changes
persist for at least 4 months after birth. Biochemical
studies have shown that a single administration of Selank
(300 μg/kg) leads to significant changes in the levels of
noradrenaline, dopamine, serotonin, and their metabolites
in the rat brain. The duration of these changes is compa-
rable to the duration of the effect of Selank on the be-
havior of the animals. Thus, the action of Selank on the
integrative brain activity of rats subjected to antenatal
hypoxia is accomplished by restoring the normal balance
of monoaminergic systems, which is accompanied by the
compensation of behavioral disorders [170,171].
The positive effects of Selank on the cognitive proc-
esses resulting from a damaged catecholaminergic sys-
tem of the brain in the early stages of ontogeny have
been reported. Administration of Selank (300 μg/kg) in
the early stages of learning facilitates the process and has
a positive effect on the preservation of these skills. The
administration of Selank to animals with reduced activity
of the catecholaminergic system is accompanied by an
improvement in the process of consolidating a memory
track and expedites the formation of a conditioned pas-
sive avoidance reflex [172].
The investigation of the activity of Selank to remedy
the alterations in the processes of learning and memory
caused by damage of the noradrenalinergic system
showed that the main component of the Selank effect is
the stimulation of the search reflex aimed at a different
adaptive response. Experiments performed using Wistar
rats with destruction of the noradrenalinergic system of
the brain caused by the disulfiram 6-hydroxydopamine
and exposure to hypoxia with hypercapnia established
the protective properties of Selank. It was shown that
Selank compensates for the learning and memory dam-
age that is associated with exposure to these neurotoxic
factors. The acceleration of the consolidation and repro-
duction processes suggests that Selank stimulates the
motivation brain mechanisms that are responsible for this
damage [173].
The effect of Selank on the processes of learning and
memory and on the level of serotonin (5-HT) was inves-
tigated in animals that received 30 training sessions in 1
day and were conditioned to food reinforcement. Selank
(300 μg/kg) was administered after the tenth run; subse-
quently, the 30-min training sessions were continued.
Preservation of the reaction was tested after 24 h, 7 days,
and 30 days. It was established that a single dose of Se-
lank causes a significant enhancement in serotonin me-
tabolism in the hypothalamus and caudal brain stem and
improves the stability of memory traces for 30 days.
These data suggest that the administration of Selank dur-
ing the consolidation phase improves the process in-
volved in saving memory traces. The nootropic activity
of Selank is due to its action at the level of serotonin and
its metabolites in the brain [174].
Given the involvement of biogenic amines in the
regulation of biorhythmological processes, the seasonal
effect of Selank on the behavior of hibernating animals
was investigated. Long-tailed ground squirrels were di-
vided into three groups according to the different stages
of the seasonal cycle: the final stage of sleep, the period
of maximum activity, and preparation for hibernation. A
single injection of Selank into the animals in the first
group exerted an activating effect of orienting-explora-
tory behavior in the “open field” test. In the second group,
no significant differences were identified between the
test animals and the controls. The third group exhibited
an increase in research activity after Selank injection.
Locomotor activity did not change in any of the groups.
Thus, the Selank psychotropic effect on animal behavior
was markedly dependent on the phase of the annual cycle.
A selective effect of Selank on animal behavior was
found exclusively in the spring and autumn periods, at a
time when hibernating animals exhibit an imbalance in
monoaminergic systems [175].
The effects of Noopept and Selank on inhibitory syn-
aptic transmission in hippocampal CA1 pyramidal cells
were investigated using the patch-clamp technique in
whole-cell configuration. Bath application of Noopept (1
μM) or Selank (2 μM) significantly increased the fre-
quency of spike-dependent spontaneous mIPSCs, whereas
spike-independent mIPSCs remained unchanged. It was
suggested that the effect of both peptides was mediated
by the activation of inhibitory interneurons terminating
on CA1 pyramidal cells. The results of recent clamp re-
cording in inhibitory interneurons residing in the stratum
radiatum confirmed this suggestion, at least for Noopept
[176].
4.2.3. Immunomodulatory Effect
Compared with the original tuftsin molecule, the effect of
the final product, Selank, on the immune system is weak-
er; however, this peptide has a strong and long-lasting
impact on the CNS [148,177].
The antiviral activity of Selank against the influenza A
virus (H3N2) was studied using in vitro and in vivo sys-
tems. Both systems revealed the presence of an antiviral
effect for the drug. The introduction of Selank in vivo
induced the expression of the Ifnα gene, without affect-
ing the expression of the Il4, Il10, and Tnfα genes. The
mechanism underlying the antiviral action of Selank is
probably linked to its ability to modulate the balance of
the Th1/Th2 cytokines [178,179].
In the clinic, it was shown that Selank completely in-
hibited IL6 gene expression (elevated IL6 mRNA levels
were observed in patients suffering from neurological
diseases) in patients with generalized anxiety disorder
and neurasthenia, whereas it had no significant effect on
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the expression of this gene in healthy patients. An in vivo
study found no marked induction of cytokines after Se-
lank administration. The adequacy of the immune re-
sponse to antigenic stimulus was determined by the bal-
ance between cell-mediated (Th1) and humoral (Th2)
responses. Significant change in the Th1/Th2 balance
was observed in both groups of patients. The lympho-
cytic index decreased significantly as a result of Selank
therapy. In contrast, Selank had the opposite effect on the
index of monocytes. The dynamics of these changes ex-
hibited a significant inverse correlation, which clearly
implies the development of a series of defense reactions
aimed both at preventing the negative effects of inter-
ferons on the nervous tissue and at preserving the active
cell-mediated immune responses [180].
Studies have shown that glyprolines have a specific
physiological action. Thus, it is supposed that synthetic
regulatory peptides, such as Semax and Selank, have
hybrid physiological properties that combine the proper-
ties of their structural components [181]. The study of
the antiviral properties of the structural fragments of the
Selank peptide allowed the selection of Gly-Pro as the
minimum amino acid sequence (pharmacophore) that had
a pronounced antiviral effect. Among the fragments of
Selank studied, the tetrapeptide Arg-Pro-Gly-Pro exhib-
ited the highest antiviral activity against the human in-
fluenza A/Aichi2/68 virus (H3N2), the human influenza
B/Ohio01/05 virus, the avian influenza virus (H5N1), the
herpes simplex virus types 1 and 2 (HSV-1 and HSV-2),
the cytomegalovirus (CMV), and the murine encephalo-
myocarditis virus (EMCV) [182].
4.3. Pharmacokinetics
Primary biodegradation of Selank occurs under the in-
fluence of the dipeptidyl carboxypeptidases, which are
responsible for the formation of the pentapeptide Thr-
Lys-Pro-Arg-Pro and the longest-lasting tripeptide Thr-
Lys-Pro and dipeptides Arg-Pro and Gly-Pro; these pep-
tides seem to have largely Selank-like effects. The sub-
sequent decay continues until the degradation to individ-
ual amino acids, the fate of which in the body is no dif-
ferent to that of the naturally occurring amino acids
[183].
Selank evenly labeled with tritium was used to com-
pare the distribution of its metabolites in tissues of rats
after intraperitoneal and intranasal administration. Pep-
tides were detected in the bloodstream from the first
minute after administration of the drug using both meth-
ods. Subsequently, the peptide content decreases rapidly,
by more than one-third at 7 min after intraperitoneal in-
jection of the drug and by half after intranasal admini-
stration of the drug, because of their rapid degradation
and distribution. The content of Selank and its metabo-
lites in most organs after intranasal administration of the
drug was significantly lower than that observed after
intraperitoneal injection. The content of Selank in the
brain after intranasal administration was comparable to
that observed in the blood, remained quite stable, and
declined more slowly than it did in the blood and in the
organs of the abdominal cavity. In vitro experiments re-
vealed that the half-life of Selank is ~2 min [183].
Thus, intranasal administration of Selank elicits rapid
penetration of the drug into the blood-brain barrier
(which allows the use of low dosages) and high bio-
availability (92.8% of active substance), and contributes
to the rapid onset of clinical effects: it is detected in the
plasma 30 s after administration and is observed in brain
tissues 2 min after administration. Selank accumulates in
the archicortex and diencephalon (target areas of the ac-
tions of Selank) and acts continuously for 20 - 24 h [183].
4.4. Effect of Selank on the Transcriptome
As noted earlier, Inozemtseva et al. found an effect of
Selank on Bdnf gene expression in the hippocampus of
rats. Administration of Selank at doses of 250 and 500
μg/kg increased the levels of the Bdnf mRNA 3 h after
injection and the levels of the BDNF protein 24 h after
injection at both doses [169].
Kolomin et al. studied the effect of Selank on gene
transcription in the rat hippocampus using cDNA mi-
croarrays (SBC-R-RC-100-13 Rat 12K cDNA v.1.3;
Shanghai BioChip, China). The microarrays contained
11,060 sequences that were homologous to the rat tran-
scriptome. Selank administration led to a change in
mRNA content (by 2 or more times) of 50 genes. Con-
comitantly, administration of a single intranasal dose of
Selank (200 μg/kg) changed the mRNA levels of 36
genes, and a curative administration of Selank (200
μg/kg once a day for 5 days) changed the mRNA levels
of 20 genes. The expression level of six genes (Actn1,
Cx3cr1, Fgf7, Kng1, Ptprn2, and Slc6a20) changed after
both single and curative introduction of Selank [184].
Genes that exhibited altered expression in the hippo-
campus of rats by more than 2-fold were clustered ac-
cording to the structure and biologic functions of their
encoded proteins using the bioinformatics software
DAVID [185]. These molecules were allocated to three
interrelated and overlapping clusters. First, the largest
cluster comprised 33 genes that encode proteins embed-
ded in the plasma membranes of cells or their organelles:
Actn1, Arfgap1, Atp5a1, Bcam, Cacna1g, Clcnka,
Cnksr2, Cx3cr1, Cxcl12, Gldn, Gpr1, Gpr 85, Gria4,
Grid2, Kcnj4, Lyn, Map2k1, Npr2, P2ry4, Pla2g16,
Ptprn2, Scamp5, Scn3b, Selp, Slc1a2, Slc5a7, Slc6a20,
Slc8a3, Smad3, Tomm20, Trp c1, Ugt1a6, and Unc13c.
Of these, 24 encode transmembrane proteins exclusively
(ion channels, transporters of ions and biologic mole-
cules, and transmembrane receptors involved in the
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processes of accumulation and transfer of energy): Bcam,
Cacna1g, Clcnka, Cx3cr1, Gldn, Gpr1, Gpr85, Gria4,
Grid2, Kcnj4, Npr 2, P2ry4, Pla2g16, Ptprn2, Scamp5,
Scn3b, Selp, Slc1a2, Slc5a7, Slc6a20, Slc8a3, Tomm20,
Trpc1, and Ugt1a6.
The second cluster included nine genes that encode
proteins associated with the membranes of neurons and
their processes: Actn1, Ca cn a1g, Gria4, Map 2k1, Kcnj4,
Klhl24, Ptprn2, Sipa1l1, and Slc1a2.
The third cluster included 15 genes encoding proteins
involved in the cellular transport system: Arfgap1,
Atp5a1, Cac na 1g, Clcnka, Gria4, Grid2, Kcnj 4, Scamp5,
Scn3b, Slc1a2, Slc5a7, Slc6a20, Slc8a3, Trpc1, and
Tomm20. Of these, 10 are involved exclusively in ion
transport and ion homeostasis for cell support: Atp5a1,
Cacna1g, Clcnka, Gria4, Grid 2, Kcn j4, Scn3b, Slc5a7,
Slc8a3, and Trpc1 [184,187].
The basic mechanism underlying the transmission of
signals, the formation of the action and resting potentials,
and the implementation of the phenomenon of long-term
potentiation and depression, which play a key role in
learning and memory formation, is a change in the level
of the balance of the intracellular Ca2+ ion [188]. Main-
tenance of ion homeostasis by the cell, primarily via ion
channels and ion transporters.
The recent study showed that the most notable in-
crease in mRNA level after the curative administration of
Selank was in the Slc8a3 gene (by 4.2 times), which en-
codes the NCX3 transporter; NCX3 plays a key role in
maintaining the sodium-calcium homeostasis of cells.
This vector provides a pathway for the introduction or
removal of calcium and sodium, depending on their in-
tracellular concentration. It was previously shown that
NCX3 is directly connected to the formation of an action
potential in cells of the hippocampus, plays a crucial role
in the formation of synaptic plasticity, and exerts neuro-
protective effects via the activation of the Akt/PKB sig-
naling pathway. NCX3 is characterized by high-speed
transport of sodium and calcium ions across the mem-
brane and plays an important role in restoring the ionic
balance disruption that results from ischemic brain dam-
age [189].
Changes in the level of mRNA were observed for
genes encoding glutamate and choline transporters. After
the administration of a single Selank dose, Slc1a2 was
downregulated 2.6-fold. This gene encodes a high-affin-
ity glutamate transporter that ensures the proper trans-
mission of nerve impulses via the removal of glutamate
from the synaptic cleft [190]. Curative Selank admini-
stration led to a decrease (by 2.9-fold) in the mRNA lev-
els of the Slc5a7 gene, which encodes a highly specific
choline transporter that is responsible for the delivery of
choline to acetylcholine-synthesizing neurons [191].
Increased mRNA levels after both single and curative
Selank introduction were noted for the Slc6a20 gene,
which encodes the Na+- and Cl-dependent proline
transporter that facilitates the reabsorption of proline in
the renal tubules. According to data from the literature,
the carrier is highly expressed in microglia, the choroid
plexus, and the meninges. The authors suggest that this
transporter is involved in the supply of proline required
for collagen synthesis, which is present in large quanti-
ties in these structures, and may play a significant role in
the reconstruction of the central nervous system after it
has been damaged. Conversely, the expression of this
gene in the brain regulates the extracellular concentration
of proline in the CNS [192,193].
A peculiar response to single and curative Selank ad-
ministration was observed for the Cx3cr1 gene, which
encodes a particular protein associated with the G-pro-
tein-coupled receptor for fractalkine: a 2-fold decrease in
Cx3cr1 mRNA levels was observed after a single inject-
tion of Selank, whereas an increase of almost 3-fold was
detected after curative administration. Fractalkine has
adhesive and migratory effects on white blood cells
(monocytes and NK cells) and participates in the adhe-
sion of T cells and macrophages to the endothelium.
Fractalkine (CX3CL1) and its receptor (CX3CR1) are
predominantly expressed in the brain and exhibit tissue-
specific expression patterns: fractalkine is expressed
mainly in neurons, and its receptor is expressed pre-
dominantly in microglia, which suggests that these two
proteins participate in the interaction between neurons
and microglia during the process of inflammation [194,
195]. The interaction of the receptors of these cells with
fractalkine leads to the induction of the antiapoptotic
pathway via the activation of NF-kB and Act kinases and
contributes to neuroprotection [196].
Thus, that study showed that both single and curative
administration of Selank alters the expression of genes
involved in the maintenance of ion homeostasis in cells
and the formation of the action potential and nerve im-
pulse transmission, and ensures the processes of synaptic
plasticity. These data suggest that the indirect effect of
Selank on gene expression has a modulatory effect on the
processes that occur in the CNS and are associated with
learning and memory formation, and has a positive
therapeutic effect in restoring the function of brain cells
that were damaged or lost as a result of ischemic or neu-
rodegenerative insults [187].
The expression of five genes (Actn1, Cx3cr1, Fgf7,
Ptprn2, and Slc6a20) that were altered in the rat hippo-
campus was analyzed via real-time PCR after the ad-
ministration of Selank to the several the brain regions
using the schemes described above. The changes in the
mRNA levels of these genes exhibited a similar pattern
in the frontal cortex and cerebellum, which, in most cases,
was different from the pattern observed in the hippo-
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A New Generation of Drugs: Synthetic Peptides Based on Natural Regulatory Peptides 239
campus. An increase in Slc6a20 mRNA levels was ob-
served in the rat hippocampus and frontal cortex after
both single and curative administration of Selank. Sig-
nificantly higher levels of the Cx3cr1 mRNA were de-
tected in the frontal cortex and cerebellum after a single
Selank injection, whereas multidirectional changes in the
mRNA level of this gene were observed in these regions
of the brain after curative administration of the peptide.
In the rat cerebellum, a significant decrease was observed
in the mRNA levels of the Ptprn2 gene, which is widely
expressed in the brain and is involved in the growth and
differentiation of nerves [184].
Together with its anxiolytic and nootropic actions, Se-
lank elicits a pronounced immunomodulatory activity
[179, 180]. Previously, it was shown that Selank admini-
stration to the rat brain leads to changes in the levels of
the Cx3cr1 mRNA, which encodes a chemokine receptor
involved in the promotion of the immune response. The
immunomodulatory effects of Selank may be due to its
ability to regulate the immune system at the level of the
transcriptome. It was shown that Cx3cr1 gene expression
after Selank introduction is considerably stronger in the
rat spleen than it is in the brain. The strongest increase in
mRNA levels was observed after a single injection of the
peptide (by 16-fold) [184].
This may indicate a more active influence of Selank on
gene expression in the spleen. Because the spleen plays a
prominent role in ensuring the protective function of the
body, such a Selank action may alter the expression lev-
els of genes that are actively involved in the immune
response and in major inflammatory reactions.
The expression of genes that are directly involved in
the process of inflammation in the spleen of mice under
the influence of Selank and some of its fragments should
be assessed in future studies.
Three Selank fragments were selected for analysis: the
natural regulatory peptide tuftsin, with a macrophage-
activating effect, the Arg-Pro-Gly-Pro peptide, which
exhibited the largest antiviral activity, and the dipeptide
Gly-Pro, which is the smallest structural Selank unit that
retains antiviral activity [182]. Selank and its fragments
caused changes in the expression of 35 genes involved in
inflammation (chemokines, cytokines, and their receptors)
in the mouse spleen at 6 h and 24 h after a single intrap-
eritoneal injection of the drug. The greatest number of
genes with significant mRNA level changes was ob-
served at 24 h after Selank injection. In contrast, Gly-Pro
led to significant changes in the mRNA levels of the
greatest number of genes at 6 h after injection. Changes
in the levels of mRNA of the greatest number of genes
encoding chemokines were observed at 6 h after admini-
stration of Gly-Pro and tuftsin and 1 day after the intro-
duction of Selank [197,198]. It is known that the frag-
ments of regulatory peptides have their own specific
physiological effects. “Hybrid” synthetic peptides com-
bine different physiological effects of the individual
fragments [181].
A similar significant decrease in the level of the
Cxcl12 mRNA (by an average of 1.3 times) was ob-
served at 6 h after the administration of Gly-Pro, Arg-
Pro-Gly-Pro, and tuftsin. The Ccr2 and Ccr4 genes en-
code chemokine receptors that are upregulated after the
introduction of the dipeptide Gly-Pro. A decrease in the
mRNA level of the Xcr1 gene was observed 1 day after
the administration of each of the peptides; this gene en-
codes the receptor of lymphotactins. A significant drop in
Itgam mRNA levels (by an average of 50 times) was
observed 6 h after the administration of Gly-Pro, Arg-
Pro-Gly-Pro, and tuftsin, whereas a decline in the mRNA
of the gene (by 2 times) was detected 1 day after the in-
troduction of Gly-Pro. There was a marked increase in
the expression of the gene encoding the interferon
gamma (Ifng; by 1.2 times) at 6 h after injection of Se-
lank and tuftsin. A significant drop in the levels of the
mRNA of the Il1r2 gene (by 14-fold) was observed 6 h
after Selank injection, and an increase in the level of the
mRNA of the gene (by 4.5 times) was detected 1 day
after the administration of the peptide. In addition, a de-
cline in the expression of the gene (by 11-fold) was noted
6 h after the administration of Gly-Pro, whereas its
upregulation (by 3 times) was observed 1 day after the
introduction of tuftsin. The Il2rg gene, which encodes a
subunit of a common receptor of various interleukins,
was characterized by a drop in the level of its mRNA 6 h
after the administration of each of the peptides [198].
Thus, the administration of Selank and all of its frag-
ments has a significant influence on the levels of expres-
sion of the mRNAs of genes encoding chemokines, cyto-
kines, and their receptors. Moreover, the activation of
some of them occurs even at 1 day after a single injection
of the peptides. As is well known, the adequacy of the
immune response to an antigenic stimulus is determined
by the balance of cell-mediated (Th1) and humoral (Th2)
responses. Moreover, Selank and tuftsin increasingly led
to changes in the expression of cytokines that are char-
acteristic of the immune response to specific cell type,
whereas the introduction of Gly-Pro altered the expres-
sion of genes involved in the humoral immune response.
Thus, we can assume that the mechanism underlying the
antiviral action of Selank is based on its ability to modu-
late the balance between Th1 and Th2 cytokines [197].
Changes in the mRNA levels of selected genes were
observed more frequently after the introduction of Gly-
Pro, and the activation of most of them was noted as
early as 6 h after injection of the peptide. Thus, we can
assume that the minimum fragment of Selank that plays a
pharmacophore role is the dipeptide Gly-Pro [187].
Among several other genes that are involved in in-
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240
flammation, it should be noted that a significant decrease
in the expression of the Ca sp1 gene, which encodes cas-
pase 1, was observed 6 h after the administration of each
of the peptides; in addition, an increase in its mRNA lev-
els was detected 1 day after the injection of Selank and
tuftsin. Changes in the expression of this gene may con-
tribute to alterations in the population of cytokines and
the maintenance of a balance of different types of cells.
Similar changes were observed in the expression of the
C3 gene, which encodes one of the components of the
complement system: downregulation of its mRNA after
the administration of each of the peptides under investi-
gation [197].
The Bcl6 gene, which encodes a nuclear sequence-
specific transcriptional repressor that plays a key role in
the formation and development of the immune system,
exhibited significant changes in its expression levels in
response to injection of each of the peptides. It should be
noted that after the administration of Selank, changes in
the mRNA levels of this gene were observed at both 6 h
and 24 h (by 4.0 and 8.3 times, respectively) after a sin-
gle injection of the peptide. The strongest decline in Bcl6
mRNA levels was observed at 6 h after tuftsin admini-
stration (by 25 times) and at 1 day after Gly-Pro and
Arg-Pro-Gly-Pro injection (by 5.3- and 16.7-fold, re-
spectively). Evaluation of the temporal dynamics of Bcl6
gene expression in the mouse spleen revealed a complex
multidirectional change of its mRNA levels after admini-
stration of Selank and its fragments. A change in the lev-
els of mRNA of the gene that was equal in value but in
the opposite direction was observed 90 min and 3 h after
Selank injection. The introduction of Gly-Pro led to a
significant decrease in Bcl6 mRNA levels at all-time
points. Introduction of tuftsin and Arg-Pro-Gly-Pro had a
negligible effect on Bcl6 gene expression. Such wave-
like changes in Bcl6 gene expression may be associated
with maintaining the balance between Th1 and Th2 cells
[197].
Thus, a possible mechanism of Selank action lies in its
ability, and the ability of its metabolites, to elicit a cu-
mulative effect on the expression of genes involved in
various biologic processes. The long-term effects of Se-
lank (stable changes in some genes, even 1 day after a
single injection of the peptide) may be due to the fact that
Selank and its metabolites act on certain specific recap-
tors that trigger a cascade of chemical reactions, which
maintain the necessary level of the original signal over a
long period.
4.5. Clinical Application
4.5.1. General i zed Anxiety Disord er and
Neurasthenia
Anxiety disorders represent a significant medical and
socioeconomic problem, and are among the most com-
mon presentations in the population of individuals with
disorders related to mental activity. Anxiety disorders are
accompanied by deterioration of the quality of life, social
and occupational maladjustment, and, finally, disability
[199-203].
Neznamov et al. conducted a clinical and pharmacol-
ogical study of the drug “Selank 0.15%” in patients with
a simple structure of anxiety and anxiety-asthenic disor-
ders and in patients with complex anxiety-phobic and
anxious-hypochondriac disorders. The administration of
the solution “Selank 0.15%” at a daily dose of 2700 μg as
early as 1 - 3 days after the beginning of therapy led to a
rapid reduction of anxiety, emotional and muscular ten-
sion, and anxious thought, as well as improvement of
mood, the appearance of vigor, increase in the total tone,
and significant reduction in fatigue in patients with gen-
eralized anxiety disorders and neurasthenia. A decrease
in irritability, sleep disorders, and the algic component of
anxiety (tension headaches and body aches) was also
noted. Normalization of sleep and a decrease in the se-
verity of autonomic disorders were also observed [204].
In patients with a complex structure of anxiety disor-
ders (anxiety-phobic and anxious-hypochondriac disor-
ders), the drug elicited a gradual reduction of symptoms
and fragmentation, with a predominant decrease in anxi-
ety and irritability. Weakening of asthenic symptoms and
drowsiness were detected, to a lesser extent, and re-
flected the stimulatory component of Selank. The mani-
festations of more complex disorders remained nearly
intact [205].
It should be noted that the implementation of a thera-
peutic regimen of Selank in patients with anxiety-as-
thenic disorders exhibits specific features. Two variants
of therapeutic dynamics of the state were observed after
administration of the drug: 1) rapid and concomitant re-
duction of the full range of psychiatric disorders with
critical dynamics in 40% of patients, and 2) gradual (lytic)
regression of symptoms in 60% of patients. At the criti-
cal dynamics more often in patients with neurasthenia,
after the first dose there was a rapid decrease in the con-
cordant whole range of psychopathological disorders—
the severity of anxiety and emotional stress, combined
with the rise of mood, energy, higher general physical
tone, and normalization of sleep [205].
Estimates of the in vitro rate of hydrolysis of labeled
Leu-enkephalin in the serum of patients showed that the
pretreatment value of τ1/2 of the peptide in patients with
generalized anxiety disorder was significantly lower than
that observed in healthy controls and was correlated with
disease duration. Selank treatment results in an increase
in the mean Leu-enkephalin τ1/2 in the serum of all pa-
tients. An increase in this parameter occurs mainly in
patients with generalized anxiety disorder, whereas an
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A New Generation of Drugs: Synthetic Peptides Based on Natural Regulatory Peptides 241
increase in the Leu-enkephalin τ1/2 value in the serum of
patients with neurasthenia as a result of Selank treatment
occurs in only half of the cases. This is because the struc-
ture of the leading states in generalized anxiety disorder
includes symptoms of anxiety and neurasthenia-asthenic
disorders, and the spectrum of the drug is determined
primarily by the anxiolytic effect [206,207].
4.5.2. Cognitive Disorders of Organic Origin
Teleshova et al. studied the neurometabolic properties of
Selank in elderly patients with disorders of vascular ori-
gin that are psychoorganic to the presence of clinical
neurosis and cognitive disorders, suggesting a possible
future deliberate application of Selank to the treatment of
cognitive disorders of organic genesis. The drug was
applied intranasally to patients over the age of 50 years at
a dose of 2700 μg per day over 14 days.
The effect of Selank became noticeable from the first
days of therapy in the form of reduced internal stress,
irritability, and anxiety, and the appearance of vigor, im-
proved mood, and increased resistance to stress. Patients
became lively and their facial expressions acquired vital-
ity and diversity. There was a decrease in asthenic mani-
festations. After 2 weeks of treatment, the majority of
patients exhibited an improvement regarding the ability
to concentrate and reduced forgetfulness.
The analysis of formalized data on the dynamics of
psychiatric symptoms in the patients studied indicated
the presence of significant changes under the influence of
Selank regarding performance anxiety, irritability, fati-
gability, and apathy. It also revealed positive dynamics in
sleep disorders, muscle hypotonia, and orthostatic disor-
ders. These results indicate that the anxiolytic effect of
Selank is combined with psychoactive effects, which
results in a reduction of asthenic disorders and normali-
zation of mental activity.
There was a positive influence of Selank on the psy-
chophysiological state of patients with this pathology, in
the form of an increase in the speed of sensorimotor re-
actions, and improvement in the parameters of attention
and in the level of short-term visual memory. In contrast
to the typical action of psychostimulants, the increase in
the reaction rate was accompanied by improvement in
the quality of these characteristics of their performance,
which is typical of neurometabolic drugs.
These results suggest complex neurometabolic, anxio-
lytic, and stimulating activities for Selank, which result
in the reduction of asthenic disorders and normalization
of mental activity. The high efficacy of the drug, together
with a significant increase in its effectiveness with in-
creased duration of the therapy, was demonstrated. These
data suggest that longer courses of therapy are required
to achieve a more pronounced effect of Selank [208].
5. Conclusions
According to the results of preclinical and clinical studies
of Semax and Selank, it can be concluded that both pep-
tides are safe and highly effective, and that drugs derived
from them are effective for the treatment of patients with
different pathologies of the CNS, as well as for the pre-
vention of these diseases and of increasing stress in
healthy people. However, given that the full spectrum of
the biologic activity and mechanisms of action of these
peptides is not fully understood, it can be assumed that
the therapeutic potential of drugs derived from Semax
and Selank has not been exhausted.
Intensive research efforts have been aimed at studying
the influence of Semax on pathological conditions that
are not associated with damage to the nervous system.
For example, experiments conducted by using laboratory
animals have shown a positive effect of Semax on the
course of acute pancreatitis in rats. A single intraperito-
neal injection of the drug at a dose of 100 μg/kg reduced
the mortality rate of animals, hyperfermentation, the ac-
tivation of lipid peroxidation, and vascular permeability,
and improved microcirculation and accelerated the heal-
ing of the zones of destruction of the pancreas [209]. The
study of the effects of Semax on various models of ul-
ceration showed a marked antiulcer effect of the drug
[210-212].
Investigation of the effect of Semax on the hemostatic
system revealed that the peptide interacts with high-mo-
lecular-weight heparin, which leads to the formation of a
complex compound with anticoagulant and fibrinolytic
properties in conditions in vitro, and in vivo when ad-
ministered intravenously. The authors suggest that Se-
max, especially in combination with heparin, may be a
promising antithrombotic agent [213]. Repeated intrana-
sal administration of Semax at a dose of 1000 μg/kg in a
hypercoagulation state caused by immobilization stress
of varying degrees yielded a powerful protective antis-
tress effect, which stimulated the anticoagulation system
[214].
Volodina et al. have shown that the administration of
Semax (23 days at a dose of 50 μg/kg) to neonatally iso-
lated rats, which led to long-term changes in animal be-
havior, significantly reduced the negative effects of neo-
natal stress [215]. Thus, these results may serve as a ba-
sis for extending the clinical use of the drug, particularly
among children in the early postnatal period.
The analysis of the results obtained in experiments
performed by using animals suggests a possible positive
effect of Semax in the treatment of diseases such as at-
tention deficits accompanied by hyperactivity and Rett
syndrome [216].
Extensive research of Selank also showed that, in ad-
dition to the above-described properties, the peptide ex-
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A New Generation of Drugs: Synthetic Peptides Based on Natural Regulatory Peptides
242
hibits several other characteristics. In various experi-
mental models of gastric ulcers, Selank and its deriva-
tives had a significant antiulcer effect by increasing the
resistance of the gastric mucosa to the factors that con-
tributed to the formation of ulcers. This represented an
application as a prophylactic drug, and also a pronounced
therapeutic effect regarding the formation and treatment
of ulcers. The study showed that the most potent antiul-
cer effect of Selank and its fragment was observed for
the hexapeptide Lys-Pro-Arg-Pro-Gly-Pro [217-219].
Selank and its biologic degradation products also have
anticoagulant, fibrin-depolymerization, and antiplatelet
properties, and do not cause bleeding complications,
even in the case of an overdose. Therefore, together with
its use as an anxiolytic and nootropic drug for the treat-
ment of generalized anxiety disorder and neurasthenia,
Selank can also be used to improve the rheological prop-
erties of the blood in many cardiovascular diseases,
cerebral circulatory disorders, diabetes, and atherosclero-
sis [220].
The direction and specificity of the peptides described
regarding the cellular transcriptome raise the possibility
of using this machinery as a pharmacological target for
the normalization of the function of cellular structures in
various pathological conditions.
6. Acknowledgements
This study was supported in part by the Russian Founda-
tion for Basic Research grant 13-04-01582 and 13-04-
40083-Н; the Russian Academy of Sciences programs:
“Molecular and Cellular Biology”, “Basic Sciences for
Medicine”, “Mechanisms of integration of molecular
systems in the implementation of the physiological func-
tions”; the Leading Scientific Schools supporting pro-
grams-SS_4294.2012.4, SS_2628.2012.4; Governmen-
tal contract (agreement #8851).
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